Many parts of an aircraft are held together by mechanical fasteners, such as rivets or bolts, as opposed to welds or other "permanent"-type connections. Mechanical fasteners can be removed so that the aircraft parts can be disassembled when, and if, the need arises. Typically, these fasteners are formed of a head on one end of a shaft. In most cases, the shaft is a substantially cylindrical shape and the head is a frustoconical shape. Accordingly, the aircraft part must have a countersunk hole for receiving the fastener. That is, the countersunk hole must have a hole for receiving the shaft of the fastener and a countersink for receiving the head of the fastener.
It is critical to the aircraft manufacturing industry that a fastener fit properly into a countersunk hole. Properly fitting fasteners provide structural integrity to the aircraft by securely holding parts together. In addition, where a fastener is inserted into an external part, such as the skin of the aircraft, a fastener must sit flush with the surface of the part so that the aerodynamic qualities of the part surface are maintained. Conversely, the structural integrity and aerodynamic surfaces of the aircraft may be impaired by fasteners that do not fit properly into countersunk holes.
The hole and countersink must have appropriate geometries to ensure the fastener will be properly received. The hole should have a cylindrical shape and have a diameter substantially equal to the shaft diameter. Likewise, the countersink should have a frustoconical shape and have a depth and diameter substantially equal to the height and diameter of the head. Clearly, assuming the fastener is made to specifications, if the countersink is the wrong shape or size, the head of the fastener may not sit flush with the part surface. Likewise, if the hole is the wrong shape or size, the fastener may not fit into the hole at all, or there may be an unacceptable amount of "play" between the part and the shaft of the fastener. In any event, the structural integrity and aerodynamic surfaces of the aircraft may be adversely affected by an ill-fitting fastener.
In addition to having the proper geometry, the hole and countersink must also have the proper orientation with respect to each other and the aircraft part to ensure the fastener will fit properly. In most cases the shaft and head of a fastener are coaxial. To ensure a proper fit, the hole and countersink of a particular countersunk hole must also be coaxial. Furthermore, to ensure the head sits flush with the part surface, the hold and countersink should be normal to the part surface. Accordingly, if the hole and countersink are not coaxial, or if they are coaxial, but are not normal to the part surface, the fastener may not fit properly into the countersunk hole and, as a result, the structural integrity of the aircraft and the aerodynamic surfaces may be adversely affected.
There is no known apparatus presently available in the aircraft manufacturing industry for automatically inspecting a countersunk hole to determine whether the geometry, as well as the orientation of the hole and countersink are within engineering specifications. Current industry practice calls for manufacturing personnel to manually insert a fastener into a countersunk hole and make a visual inspection of the fit. If the fastener does not appear to fit properly, e.g., the head of the fastener is not flush with the part surface, or the shaft does not fit into the hole, the hole and/or countersink may be redrilled in an attempt to correct the problem. Visual inspection is a relatively inexact method of inspection that is subject to errors and oversights, which may threaten the structural integrity and aerodynamic performance of the aircraft. It is desirable to detect these problems early in the maufacturing process because the cost of correcting the problems is less than when the problems are detected, and corrected, later in the manufacturing process. The inaccurate nature of visual inspection may result in these problems being overlooked, which, in turn, may result in more costly corrections later in the manufacturing process. Considering the large number of fasteners that are inserted into countersunk holes in an aircraft, the cost and opportunity for error with such a method become even more significant.
As a result, there has developed a need in the aircraft manufacturing industry for an apparatus that will automatically inspect a countersunk hole in conductive aircraft part and determine whether the geometry and the orientation of the countersunk hole are within engineering specifications. The present invention provides a capacitance probe having a novel configuration that permits the probe to be inserted into a countersunk hole. A fringe field capacitance technique is used to measure the distance between the probe and the walls of the hole and countersink of a countersunk hole. The probe provides adequate measurement information to permit another device, which does not form a part of this invention, to compute the geometry and orientation of the countersunk hole and determine if the countersunk hole is within engineering specifications. An example of such a device can be found in a contemporaneously filed U.S. patent application Ser. No. entitled "Method and Apparatus for Measuring the Distance Between a Body and a Capacitance Probe," by Dennis P. Sarr and Patrick L. Anderson, application Ser. No. 07/320,315, the subject matter of which is incorporated herein by reference.